Recirculating type steam generator and pump arrangement



Se t. 16, 1969' c. J. GARRETT. JR 3,467,069

RECIROULATING TYPE STEAM GENERATOR AND PUMP ARRANGEMENT Filed Dec. 28, 19a? 2 Sheets-Shet 1 IA/jfi' [32 rs EM sgs TA NC E "7* E W E moo 2000 3000 4000 5000 6000 7000 8000 FLOW 6AL5./M/N.

INVENTOR CARLOS J GARRETT .12.

,4 T TOENE Y p 6, 1969 c. J. GARRETT, JR 3,467,069

RECIRCULATING TYPE STEAM GENERATOR AND PUMP ARRANGEMENT Filed Dec. 26, 1967 2 Sheets-Sheet 2 INVENTOR. CAAZas 7- GA/M'FT 7n.

3,467,069 RECIRCULATING TYPE STEAM GENERATOR AND PUMP ARRANGEMENT Carlos J. Garrett, Jr., Granby, Conn., assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Dec. 26, 1967, Ser. No. 693,423 Int. Cl. F22d 7/08, 7/12 U.S. Cl. 122-406 11 Claims ABSTRACT OF THE DISCLOSURE A supercritical pressure through-flow steam generator with recirculation through the furnace wall tubes. Two centrifugal pumps to effect recirculation are located either in the recirculating line or in the portion of the through-flow path through which recirculation is to be effected. Either of these pumps is satisfactory to meet the normal needs of the steam generator while the other is a spare. The pumps are arranged so that they may be operated in series flow relations during certain critical periods of operation.

BACKGROUND OF THE INVENTION This invention relates to through-flow steam generators of the recirculating type and in particular to a circulating pump arrangement therefor.

Supercritical pressure steam generators operate on the through-flow principle where a feedwater pump forces water serially through tubular heating surface within the steam generator and then to a steam turbine or other steam consuming device. The water generally first flows through an economizer, then through the furnace Wall tubes and finally through the superheater. In the recirculating type unit a portion of the water is withdrawn downstream of the furnace wall tubes and recirculated to a location upstream of the furnace wall tubes. This provides higher flow rates through the furnace wall tubes during low load operation than would normally be experienced. This high flow, in turn, decreases metal temperature of the tubes due to the lower resistance of the tube-to-water film-to-heat transfer. It also decreases the temperature difference of the fluid passing through the waterwall tubes. The circulating pump effecting such recirculation may be located either in the recirculating line or in the through-flow portion through which recirculation is to be effected. U.S. Patents 3,135,252 and 3,135,- 249 describe the operation of such a system where the circulating pump floats on the recirculating system.

At any given load on the steam generator, the temperature of the fluid leaving the furnace walls remains constant regardless of the amount of recirculation. The inlet temperature to the walls varies along with the flow quantity through the walls. When the unit is started up With cold water, the entire recirculating loop is, of course, at low temperature. The unit is heated up and a small through-flow is started such that a point is reached where a high temperature in the order of 800 F. is reached by the fluid leaving the waterwall tubes. A large quantity of this fluid is recirculated and mixed with low temperature incoming feedwater so that the tempearture of the fluid entering the furnace walls is only slightly less than that leaving the furnace walls. As load is increased on the steam generator, the through-flow increases while thte recirculating flow decreases resulting in a measure of more cold water and less hot water whereby the inlet temperature to the waterwalls decreases while the water wall outlet temperature remains genrally at the same level as before.

United States Patent At low load where high temperature exists at the water- Wall inlet, the water has a high specific volume. Therefore while a high volumetric flow rate is achieved, the mass flow rate is relatively low. Since mass flow rate is a major determinative factor on the resistance of the inside tube film to heat transfer, this low density leads to relatively high tube metal temperatures. Increasing the mass flow rate would, of course, provide some relief.

Since a furnace wall is comprised of a large number of tubes in parallel, a problem exists in distributing flow between the many parallel tubes. Where there is a flow maldistribution, it is those tubes receiving a lower percentage of the flow which produce high tube temperature problems. This tube-to-tube stability is improved when the change in density of the fluid passing through the furnace wall tubes is minimized. Therefore higher recirculation rates producing a higher inlet temperature tend to improve the tube-to-tube stability. Furthermore, where insufficient pressure drop exists through the furnace wall circuits, problems are sometimes encountered with the weight of water in the tube which receives very little heat. The weight of water in this tube is reduced as the density decreases with increased temperature, and therefore this problem is also minimized by increased recircu lation.

The problems of low mass flow and poor distribution are most difficult at loads in the order of 20 percent or less of full load rating where relatively high temperature low density fluid enters the WaterWalls. Higher recirculation rates at this time would be advantageous. The circulating pump is normally supplied with a spare which is arranged in parallel, and these two pumps are operated in parallel in an attempt to increase the flow rate. However since the temperature of the mixed fluid entering the waterwells increases with recirculation, the mass flow increase achieved by operating the two pumps in parallel is in the order of 1 to 2 percent.

SUMMARY OF THE INVENTION In my invention the conventionally supplied recirculating pump and spare are arranged so that they may be operated in series flow realtion. The increase in mass flow rate achieved through the furnace wall tubes by so arranging these pumps is about 10 percent rather than the 1 to 2 percent of the prior art parallel flow arrangement. Either of the two pumps may be opearted alone with the other isolated or the two pumps may be operated in series.

It is an object of my invention to maximize the recirculation through the furnace wall tubes during critical periods of steam generator operation without the installation of additional pumping capacity.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic diagram of a steam generator with the recirculating pumps in the mixed flow portion of the flow path;

FIGURE 2 shows the interrelation of the system resistance curve and the pump curves for parallel and series arrangement of recirculating pumps;

FIGURE 3 is a schematic diagram of an arrangement of circulating pumps generally the same as the arrangement shown in FIGURE 1;

FIGURE 4 is an arrangement similar to FIGURE 3 with a slightly different valve arrangement;

FIGURE 5 is a schematic diagram of a pumping arrangement which uses less valves but precludes parallel operation of the circulating pumps; and

FIGURE 6 is a schematic diagram of a steam generator with the circulating pumps being located in the recirulating line.

3 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGURE 1, feedwater pump '21 forces feedwater through the steam generator. The steam generator includes economizer surface 22, furnace waterwall surface 23 and superheater 24. The heated through flow is then passed through steam turbine 25 and condensed in condnser 27. The waterwall surface 23 is comprised of parallel tubes lines the walls of furnace 28 with this furnace including burner 29 firing fuel into the furnace.

A flow division means 30 which may be a simple pipe T is provided, with recirculating line 32 containing check valve 33 being arranged to convey recirculated fluid back to a mixing vessel 34. Recirculation through this system is effected by circulating pump 35 which is located in the mixed flow portion of the through-flow path.

The system resistance curve on FIGURE 2 is a plot of the over-all resistance of the recirculating path when the steam generator is operated at a 20 percent through-flow. This system resistance is much steeper than a square curve for several reasons. The net hydraulic differential between the downcomer portion of the loop and the riser portion of the loop is effective to produce a pumping action through the loop. Therefore, the system resistance curve tends to approach a value less than zero as it approaches zero flow. The curve is also steep because it must be plotted against the head in feet of the fluid being pumped by pump 35 in order to produce a meaningful curve in relation to the pump curve. As the recirculation is increased at a given load, the temperature of the mixed water passing through pump 35 increases. The density therefore decreases and any given pressure drop is converted into feet of this fluid and a relatively high figure is obtained for the high recirculation rates. If this particular measurement of head were not used, the application of the pump curve to the system resistance curve would become very diflicult, but obviously the same results will ultimately be produced.

Curve 41 is a pump curve representing the characteristic of pump 35. With only this pump operating, the actual flow through the system is approximately 6000 g.p.m. as represented by point 42 at the intersection of the pump curve and the system resistance curve. With this operation, valves 2 and 3 must be opened.

Conventional parallel operation of two pumps results in the operating of pump 43 in parallel with pump 35 by also opening valves 1 and 5. The composite pump curve of these two pumps operating in parallel is represented by curve 44 which intersects the system resistance curve at point 45. It can be seen that the increase in flow obtained is only nominal; and when this is corrected for the decreased density at this higher flow rate, the increase in mass flow is only 1 to 2 percent.

In accordance with my invention pumps 35 and 43 may be operated in series by opening valve 4. The check valve feature of valve 1 prevents reverse flow to the suction of pump 35 while the check valve feature of valve 2 prevents reverse flow from the discharge of pump 43. These two pumps operating in series operate in accordance with pump curve 46. This curve intersects the system resistance curve at point 47 representing a flow of about 7300 gallons per minute. Since the density of the fluid is decreased, the resultant increase in mass flow is only in the order of 10 percent, but this is substantially greater than the increase achieved by parallel operation of the two pumps.

Since the changeover from one pump operation to operation of two series pumps may produce a substantial temporary disturbance in the steam generator, the two pumps may be first operated in parallel with valve 4 remaining closed. Valve 4 may then be gradually opened so that the change from parallel operation to series operation may. be gradually accomplished without a sudden disturbance in the steam generating system.

FIGURE 3 illustrates the same valve arrangement as used in FIGURE 1 but with a slightly different schematic arrangement. It can be seen from either of these illustrations that pump 35 may be isolated to perform maintenance work or as a temporary expedient in the event of a leak by closing valves 3, 4 and 2. All flow then passes through valve 1 and circulating pump 43. Alternately, pump 43 may be isolated by closing valves 1, 4 and 5 with pump 35 operating. Many units are designed so that during high load operation circulating pump operation is not required. With the valve arrangement of FIGURES l and 3 such operation can be accomplished by closing valves 3 and 5 with the flow bypassing the pumps through valves 1, 4 and 2. During this operation neither pump 35 nor pump 43 can be isolated.

FIGURE 4 illustrates an alternate valve arrangement wherein valve 11 is a simple check valve in lieu of the stop check valve 1 and valve 12 is a simple check valve in lieu of the stop check valve 2. Stop valve 6 is added at the discharge side of pump 35 with stop valve 7 being added at the suction side of pump 43. With this arrangement both pumps may be bypassed with valves 6 and 7 closed permitting isolation of either or both of the circulating pumps. With such a valve arrangement pump 35 may be isolated by closing only valves 3 and 6 while pump 43 may be isolated by closing only valves 5 and 7.

FIGURE 5 illustrates an alternate valving arrangement which is simpler and less expensive than the arrangements of FIGURES 3 and 4. Again pump 35 may be isolated by closing valves 3 and 6 and pump 43 may be isolated by closing valves 5 and 7. The unit may be operated by passing flow around both pumps of valves 11 and 12, if desired. Either pump may be operated alone or the two may be operated in series. This particular valving arrangement, however, precludes parallel operation of the two circulating pumps.

FIGURE 6 illustrates a steam generator similar to FIGURE 1. The sole difference is the location of the circulating pumps which are located in the recirculating line 32 rather than in the through-flow path. The pump and valve arrangement is the same as that illustrated in FIGURE 4. Since the waterwall outlet temperature does not change with recirculation, the temperature of the fluid passing to these pumps and, therefore, its density is constant at any given load. Since the steam generator is generally operated over a substantial load range of approximately a constant waterwall outlet temperature, there is little variation in density during startup since these pumps initially operate on cold and finally approach operation on 800 F. steam.

As the load is increased on a steam generator, the pressure drop between the mixing vessel 34 and the division means 30 increases substantially. Since the pump is pumping fluid at very low density, it is very difficult for this pump to produce enough head to overcome the pressure differential across the recirculating portion of the through-flow path. The pump could, of course, be designed for a very high head. Since the pump must operate on cold water during startup at this same high head, motor power requirements would be substantial. With the series arrangement of my invention these pumps may be operated in series when required to obtain substantial recirculation at high loads in the steam generator since the pump heads are additive rather than the pump flows as generally indicated in FIGURE 2.

While I have illustrated and described a preferred embodiment of my invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is: 1. A supercritical pressure steam generator comprising:

a furnace; means for burning fuel within said furnace;

tubes lining the walls of said furnace; feed pump means for establishing a through-flow of Water at supercritical pressurefithrough said tubes; a water flow mixing means located upstream of said tubes and downstream of said feed pump means; a water flow division means located downstream of said tubes; conduit means for conveying fluid from said flow division means to said flow mixing means; amixed flow path from said mixing means to said tubes and then to said division means; said conduit means and said mixed flow path comprising a recirculating path; and pump means located in said recirculating path comprising a first centrifugal pump, 5a second centrifugal pump, said first and second pumps arranged in series flow relation.

2. An apparatus as in claim 1 having also: a first bypass conduit around said first pump of "sufiicient size to carry at least one pump flow and a' first valve in said first bypass conduit; a second bypass ;conduit around said second pump of sufficient size to carry at least one pump flow and a second valve in said second bypass conduit.

3. An apparatus as in claim 2 wherein said first and second valves are check valves.

4. An apparatus as in claim 2 having also: a third valve upstream of said first pump, a fourth valve downstream of said first pump and upstream of said second pump, said first bypass conduit also bypassing said third and fourth valves; a fifth valve downstream of said second pump, said second bypass conduit also bypassing said fourth and fifth valves.

5. An apparatus as in claim 4 wherein said third, fourth and fifth valves are stop valves and wherein said first and second valves are stop check valves.

6. An apparatus as in claim 4 having also: a sixth valve immediately downstream of said first pump and upstream of said second bypass; and a seventh valve immediately upstream of said second pump and downstream of said first bypass.

7. An apparatus as in claim 6 wherein said first and second valves are check valves.

8. An apparatus as in claim 3 having also: a third valve upstream of said first pump; a fourth valve immediately downstream of said first pump, said first bypass conduit also bypassing said third and fourth valves; a fifth valve downstream of said second pump; a sixth valve immediately upstream of said second pump; and said second bypass conduit also bypassing said fifth and sixth valves.

9. An apparatus as in claim 3 wherein said circulating means are. located in said mixed flow path between said mixing means and said tubes, and wherein said' bypass conduits are each of sufiicient size to carry full load steam generator through-flow.

10. An apparatus as in claim 7 wherein said circulating means are located in said mixed flow path between said mixing means and said tubes, and wherein said bypass conduits are each of sufiicient size to carry full load steam generator through-flow.

11. An apparatus as in claim 2 wherein said circulating means is located in said conduit means.

References Cited UNITED STATES PATENTS KENNETH W. SPRAGUE, Primary Examiner 

